1. Field of the Invention
The present invention relates generally to signal comparators.
2. Description of the Related Art
Comparators compare the magnitude of two signals and produce an output that indicates which is greater. This function is an important part of signal conditioning systems. In most analog-to-digital converters, for example, comparison is necessary to initiate the process of quantizing an unknown analog signal.
In a typical comparator process, a differential analog input signal is compared to a differential reference signal. One comparator structure directed to this purpose is the switched-capacitor comparator that presents capacitors to sample a voltage from the input signal in one operational phase and to sample a voltage from the reference signal in another phase. The difference between the voltages is stored on the capacitors and a differential amplifier compares the sampled difference to zero and its output indicates the comparison.
Although switched-capacitor comparators are accurate and are relatively insensitive to common-mode levels, they tend to reduce comparison speed because they draw currents from associated circuits. This current flow tends to inject charges into the associated circuits which disturbs them and increases settling times. In addition, the switched capacitors load the associated circuits and add complexity because they require additional timing circuits to control their operational phases.
A simpler comparator structure uses two differential amplifiers to form the difference between a differential input signal and a differential reference signal. Sometimes called a 4-input comparator, this structure exhibits improved speed because it does not require the sampling processes of the switched-capacitor comparator. Unfortunately, differences between the common-mode levels of the input and reference signals alter the amplifier gains so that 4-input comparators are sensitive to common-mode differences.
Differential amplifiers have been fabricated with device mismatches that produce an inherent offset which essentially equals the reference signal. This structure can form a high speed comparator that is relatively insensitive to common-mode differences. However, the offset is quite sensitive to several effects (e.g., temperature, fabrication processes and power supply variations) and this sensitivity has limited the usefulness of these comparators.